The present invention relates to ultrasound contrast inaging, and more particularly to apparatus and methods for coloring enhanced images.
The invention has particular application in detecting scatter behaviour from free gas bubbles which may be present in a volume to be imaged. Such free gas bubbles may be, for example, generated in a volume of interest by introduction of ultrasound contrast imaging agents.
Ultrasound contrast agents can be introduced into the body to reflect or absorb ultrasound energy, or to resonate when exposed to such energy, and thereby provide an enhanced image of a part of the body. Examples of such contrast agents, in the form of hollow microcapsules, are given in Japanese Patent Applications Nos. 508032/1992 and 509745/1994 and in PCT/GB95/02673 (WO 96/15814). Such agents are injected into a patient""s bloodstream and then the patient is subjected to ultrasound radiation.
The production of free gas bubbles from the hollow microcapsules is described in co-pending UK Patent Application No. 9800813.9.
Harmonic Imaging (HI) is therefore a well-established imaging modality for ultrasound contrast agents. Due to the nonlinear scatter behaviour an oscillating gas bubble generates multiple components of the transmitted frequency (harmonics). Under specific conditions, however, gas bubbles also backscatter subharmonics of the transmitted frequency (Neppiras; Subharmonic and other low-frequency emission from bubbles in sound-irradiated liquids. Journal Acoust Soc Am. 46: 587-601; 1996. Eller and Flynn; Generation of subharmonics of the order one-half by bubbles in a sound field. Journal Acoust Soc Am. 46 722-727; 1969). According to theory the onset of subharmonics, for a free gas bubble, depends on the transmitted frequency and the applied acoustic pressure. The acoustic pressure for the onset of subharmonics scattering is minimal at twice the resonance frequency of the gas bubble. Additionally, narrow band signals are needed because the generated subharmonic components will be more dominant when the number of periods increase.
Recently several investigators have studied the possibility for subharmonics imaging of ultrasound contrast agents (Lotsberg et al; Experimental observation of subharmonic oscillations in Infoson bubbles. Journal Acoust Soc Am. 99: 1366-1369; 1996; Shankar et al; Advantages of subharmonic over second harmonic backscatter for contrast-to-tissue echo enhancement. Ultrasound Med. Biol. 24; 395-399; 1998; Forsberg and Shi; New aspects of harmonic and subharmonic imaging. The leading edge in diagnostic ultrasound. Atlantic City; 1998). Lotsbert et al (1996) observed subharmonic scattering for Albunex. It was concluded that the acoustic pressure for the onset of subharmonics was lower than expected from the theory, as developed by Eller and Flynn (1969) where they corrected for the presence of the shell of the Albunex microspheres. The authors gave a possible explanation by assuming that because of the acoustic pressure the encapsulating shell breaks and the free air bubbles appear and will oscillate with subharmonics at a lower sound pressure. Shankar et al (1998) described the advantage of subharmonic imaging over conventional harmonic imaging concerning agent-to-tissue ratio. As function of pressure this ratio increases for the subharmonic, whereas for the second harmonic it decreases. This is probably due to the nonlinear propagation effects in tissue where second harmonic components are generated, especially at higher values of the acoustic pressure. Also, attenuation is lower for subharmonic compared to second harmonic components.
Though the harmonic nature of oscillating gas bubbles is known, subharmonics are not used in diagnostic ultrasound. This is because the characteristics are such that the resolution of subharmonic imaging is too low.
The present invention has for an object to use subharmonics in diagnostic imaging. The invention has, for a further object, to provide a new and improved transducer design and also a new and improved imaging strategy which will increase the sensitivity of the subharmonic signals and will overcome the problem of limited resolution which has heretofore prevented successful practical imaging using subharmonics.
An important advantage of using subharmonics with free gas bubbles is that the subharmonic signals are only generated from the free gas bubbles and not from any surrounding tissue and therefore an image is created of the free gas bubbles without the image from the tissue. This can be utilised to provide an enhanced image.
The present invention provides apparatus for obtaining an ultrasound image of an object, said apparatus comprising a transducer array, said transducer array including at least one first part comprising a first transducer type for transmission of ultrasound waves into the object to be imaged and at least a second part comprising a second transducer type for receiving subharmonic waves from the object being imaged, said first and second parts of said transducer being mounted adjacent to each other. Preferably each of said first and second types of said transducer comprise a plurality of transducer elements.
Preferably said apparatus further comprises means for steering the focus point of said plurality of transducer elements of said second part of said transducer to receive subharmonic signals from a plurality of points within said object.
In a specific embodiment the transducer array further comprises an additional third transducer part situated adjacent to said first transducer part on an opposite side to said second transducer part. Preferably said third transducer part is of the second transducer type.
Preferably said third transducer part is identical to said second transducer part.
Preferably said third transducer part also comprises means for steering the focus of the third transducer part to receive subharmonic signals from a plurality of points within said object.
In a further alternative embodiment, said third part is of a third transducer type, said third transducer type being sensitive to receive second harmonic signals from within said object.
Preferably said third transducer type is also steerable to focus on various points within the object.
The present invention also provides a method for ultrasonic imaging of an object, said method comprising the steps of:
i. generating a first burst of ultrasound focused into a first region of an object to imaged;
ii. receiving first subharmonic ultrasonic signals from plurality of first focus points within said first region of said object;
iii. receiving at least second subharmonic ultrasonic signals for said at least second region of said object to be imaged;
iv. storing said first and at least second subharmonic ultrasonic signals; and
v. processing said first and at least second stored signals to produce an image of said object.
Preferably said processing of said image of said object also comprises comparison of said image produced from said subharmonic signals with fundamental output signals from the same region of said object.
Preferably said method further comprises receiving second harmonic signals from said first and said at least second region of said object and storing said second harmonic signal output, said processing comprising comparing the subharmonic and second harmonic output signals to produce an enhanced image of said object.
In a preferred embodiment the first burst of ultrasound comprises two or more portions, a first preparation portion and a second imaging portion at a different amplitude or frequency from said first preparation portion. The first preparation portion comprises a pulse burst which does not generate substantial subharmonics but is of an amplitude or frequency which creates a rapid onset of subharmonics in the imaging portion.
The present invention therefore also provides a method of ultrasonic imaging of an object, said method comprising the steps of generating a burst of ultrasound focused into a region of said object to be imaged, said burst of ultrasound comprising two or more portions, a first preparation portion and a second portion at a different amplitude or frequency from said first preparation portion, said burst being designed to generate subharmonics within said object.